The present invention relates to a method of cleaning and preparing a monocrystalline semiconductor surface to be processed, i.e., the work surface of a semiconductor substrate, and particularly to a method of preparing a semiconductor surface having a surface orientation somewhat off the (001) plane.
Control of the work surface of a semiconductor substrate in the semiconductor process greatly influences the quality of semiconductor devices manufactured thereafter. For example, residual impurities on the work surface, e.g., a natural oxide film, organic contaminant and heavy metal, act as a process inhibiting factor or device performance reducing factor.
As regards an Si monocrystalline substrate, the residual impurities make it difficult to control a thin gate oxide film with high accuracy. Further, the impurities deteriorate electric characteristics such as an insulation breakdown voltage, increase the serial resistance of a metal ohmic contact, and also deteriorate rectification characteristics. Moreover, it is known that the roughness of the work surface, formed in a step of forming a thin gate oxide film, is a factor of device deterioration, such as deterioration in the mobility. Therefore, in the process of manufacturing semiconductor devices, it is indispensable to form a work surface, which can be clean and flat, free from residual impurities. The requirement for cleanness of the work surface is studied in the field of various materials as a common problem not only semiconductor.
In general, to obtain an ideal clean Si monocrystalline surface, an Si monocrystal (an ingot) is sliced, and the sliced surface is polished and chemically etched, thereby forming a mirror surface. A wafer surface immediately after slicing may often include blade marks, in which case surface roughness of from sub-micron to hundreds of microns remains. Further, surface contaminant, mainly consisting of organic substances, metal and oxide film, exists, in addition to the roughness. To remove the contaminant, the wafer is washed first. Thereafter, the wafer is chemically polished to finish the surface to roughness of several nanometers, whereby an Si wafer is obtained. On the Si wafer thus obtained, a natural oxide film is formed. Since the roughness of several nanometers exists on the wafer surface, i.e., between the natural oxide film and the Si wafer, flatness in the atomic order is not realized.
Jpn. Pat. Appln. KOKAI Publication No. 8-264780 discloses a treatment method for obtaining flatness in the atomic order in the Si wafer. In the method disclosed in this publication, after an ingot is sliced into an Si wafer or a natural oxide film is removed from the Si wafer, surface contaminants such as organic substances are removed from the Si wafer. Then, the Si wafer is subjected to a heat treatment at a temperature of 1000.degree. C. or higher in an atmosphere having a hydrogen partial pressure, thereby reconstructing the surface. As a result, a flat wafer surface including a step 8 and a terrace 9 is obtained (see FIG. 2). In other words, according to this method, the washing and high temperature treatment removes the roughness and contaminant generated as a result of slicing and reduces the surface roughness of sub-micron or hundreds of microns. The Si wafer thus produced has a natural oxide film on its surface when the user receives it; however, the wafer surface itself is clean and flat.
However, according to the conventional methods of cleaning a semiconductor work surface, including the method disclosed in the aforementioned publication, the residual impurities on the surface cannot be reduced sufficiently. Hence, deterioration in device performance, which is considered to result from contaminant on the surface, is frequently observed as process dependency. In particular, even if the wafer is cleaned by the above method, once the natural oxide film is removed from the surface during manufacture of semiconductor devices, the flatness of the substrate surface is gradually deteriorated by new contaminant and corrosion, causing deterioration in the device performance.
As a means for solving the above problem, a method of coating the work surface of an Si substrate with an oxide film by RCA washing is proposed (W. Kern, RCA Review, Vol. 31, 1970). In general, when an oxide film is formed on an Si substrate, the residual impurities, which have adhered to the Si substrate before forming an oxide film, are taken into the oxide film. Therefore, if adhesion of residual impurities on the Si substrate surface is suppressed immediately after a step of removing the oxide film subsequent to the step of forming the oxide film, a clean Si substrate can be obtained.
For example, in the case of RCA washing as described above, an oxide film having a thickness of 1 nm is formed on an Si substrate. Then, the Si substrate is introduced in a vacuum chamber which can heat a substrate. In the vacuum device, the Si substrate is heated at a temperature of 800.degree. C. or higher in an ultra-high vacuum of about 10.sup.-8 Pa, in which an Si substrate is not likely contaminated. Thus, the oxide film on the Si substrate can be removed and a clean surface can be obtained. Further, this method is advantageous in that an oxide film can be formed relatively easily, and that the oxide film also serves as a protection film for protecting the interface formed thereunder when the substrate is left in the atmosphere.
However, in a case of processing a substrate of a large diameter of 8 inches or greater, which is considered to come into wide use in the future, the heat treatment step under an ultra-high vacuum may raise problems, such as an increase in size or cost of the manufacturing device. In addition, the complexity of the treatment process may increase the possibility of substrate contamination during the process.
On the other hand, when manufacturing current fine ICs, a sophisticated manufacturing process has become indispensable to form a semiconductor device on another semiconductor device. In such a process, it is necessary to realize a lower-temperature process without the aforementioned high vacuum heat treatment, more specifically, to perform all the process of manufacturing semiconductor devices at a temperature of 800.degree. C. or lower. Thus, as regards cleaning of the substrate, there has been an increasing demand for a process by which a clean Si substrate can be obtained at a low temperature. Nevertheless, no specific means for cleaning an Si surface has been proposed, which satisfies the conditions as described above and may realize the low-temperature process.
Therefore, in a process to which the aforementioned high-temperature treatment method cannot be applied, it is necessary to remove the oxide film by another method immediately before the process. In general, HF solution treatment is employed for this purpose. An Si surface from which the natural oxide film is removed and hydrogen-terminated is by HF solution is comparatively stable in the atmosphere. This is an advantage of the treatment using HF solution.
However, the Si surface after the HF solution treatment is generally subjected to a step of washing the HF solution away by pure water in order to assure safety. It is preferable that the period of time for the washing step using pure water be as short as possible. Since oxygen in the atmosphere is dissolved in pure water to a high concentration in the ppm order, the washed surface is generally oxidized. Therefore, if the surface is washed by pure water for a long period of time, an oxide film is formed again on the surface, which has been cleaned by the HF solution.
On the other hand, it has been found that if oxygen dissolved in pure water is removed, the surface oxidation in the step of washing with pure water is suppressed, as the concentration of the dissolved oxygen is reduced. For example, as disclosed in Japanese Patent Application No. 53614/93, in a step of washing an Si substrate with ultrapure water containing oxygen in a low concentration, etching of the Si substrate with pure water progresses. As a result, in a dissolved oxygen concentration lower than a threshold value, although a part of the surface may be oxidized by a very small amount of oxygen remaining in pure water, it is considered that the surface is always kept clean, since the oxide film is removed together with the Si surface layer by etching.
Further, use of low oxygen concentration ultrapure water realizes direct reaction between pure water and Si on the substrate surface, which has been prevented by the surface oxidation (K. Usuda et al. Appl. Phys. Lett., 64,3240 (1994)). If the reaction is realized, the surface roughness of the Si substrate may increase, as the time period of washing by pure water increases (H. Kanaya et al., Appl. Phys. Lett., 67,683 (1995)).
However, since details of the change in surface roughness has been unknown, the mechanism of etching the Si substrate by pure water has not been clarified. Therefore, it is required to realize a method for reducing roughness based on clarification of the conditions of increasing the surface roughness or the mechanism of etching.
As described above, surface control for achieving both cleaning and flattening semiconductor crystal has been studied as a technique indispensable to form, for example, a semiconductor surface. However, the conventional treatment includes a number of steps, which are complicated, but a satisfactory effect has not been obtained. Therefore, the conventional treatment requires a great amount of chemicals and pure water for cleaning. Under the circumstances, it is necessary to realize a simpler cleaning process to decrease the number of steps and cost, and to conserve global environmental by decreasing the amount of chemicals. It is a problem of the cleaning process, which arises in actual device manufacture, that the residual impurities on the surface cannot be fully removed after the process and the surface may become less flat.